1. Field of the Invention
The present invention relates to a solid-state color imaging apparatus for obtaining color image signals by means of a solid-state device.
2. Description of the Prior Art
There is already proposed the use of one, two or three charge-coupled devices (CCD) for realizing a solid-state color imaging apparatus, but the present invention relates particularly to a single-chip imaging process and an apparatus therefor employing a CCD.
In the conventional single-chip solid-state imaging apparatus utilizing a solid-state imaging device such as CCD, the color separation has been achieved by superposing a color filter array in one-to-one relationship with imaging cells. The color filter array used in such single-chip solid-state imaging apparatus is generally composed of a color filter having a color mosaic pattern as shown in FIG. 1 in order to reduce the number of imaging cells of CCD. The color filter array shown in FIG. 1, is of so-called Bayer arrangement, having color filter elements 1, 2 and 3 respectively transmitting red (R), blue (B) and green (G) color components. Such color filter array has been associated with unsecure color separation due to mutual color mixing caused by signal leak between the CCD cells even if the color filter elements 1, 2, 3 are completely aligned with said cells.
Particularly in case of a frame transfer type CCD, said color mixing amounts to a considerable extent because of the structure, as will be explained in the following.
FIGS. 2(a) and 2(b) respectively show a vertical cross-sectional view of a 4-phase drive frame-transfer CCD for imaging by signal accumulation under two of four electrodes, and a chart showing the probability of charge capture.
Also FIGS. 3(a) and 3(b) respectively show a cross-sectional view in the horizontal direction and a chart showing the probability of charge capture.
In FIGS. 2(a) and 3(a) there are shown transfer electrodes 4 grouped in a set of .phi.1, .phi.2, .phi.3 and .phi.4 for 4-phase drive for each cell; an insulating layer 5; and a silicon substrate 6. 7 indicates a potential curve having potential wells 8 and potential barriers 9 formed by the voltages applied to said transfer electrodes. Also there are shown channel stoppers 10. In the illustrated state the potential well or photoreceptor cell 8 is formed in a space defined by said channel stoppers 10 and the potential barriers 9 positioned under the transfer electrodes .phi.3 and .phi.4.
FIGS. 2(b) and 3(b) show the probability distributions g(x), h(y) of capturing the charge generated by photoelectric conversion into the photoreceptor cell 8 formed by potential curve 7.
Said probability distributions g(x), h(y) indicate that the photosensitivity curves of neighboring photoreceptor cells 8 mutually overlap at the potential barriers 9 under the channel stopper 10 and under the transfer electrodes 4 (.phi.3, .phi.4), whereby a color mixing appears between the adjacent cells even if the color filter elements 1, 2, 3 are exactly aligned with the imaging cells.
Such color mixing can be eliminated with respect to the horizontal direction of a 2-dimensional CCD by rendering the channel stopper 10 opaque to light. However, with respect to the vertical direction, the transfer electrodes 4 cannot be made opaque for enabling interlaced imaging, whereby the color filter of the mosaic pattern shown in FIG. 1 results in an inevitable color mixing as high as approximately 25%. Since such color mixing can only be eliminated by a considerably complicated correcting circuit, it is not practically acceptable to combine the frame--transfer CCD with filter of the Bayer arrangement as shown in FIG. 1.
Also the color camera using CCD has generally been associated with a poor sensitivity, as the electrodes .phi.1, .phi.2, .phi.3 and .phi.4 have usually been prepared with polysilicon having a poor transmission to blue component.
Furthermore, as the solid-state color imaging apparatus functions in the normal operation at approximately 1/8 of the saturation level of the imaging device, an incident light of the saturation level can be regarded as a strong white light, whereby the corresponding signal is generally white clipped by the signal processing circuit. However, in case of the single-chip apparatus, the coded output signals Y, R and B assume the behavior as shown in FIG. 3(b) whereby the output signals R and B decline in response to incident light exceeding the saturation level of the cells corresponding to (Ma +G). Consequently the white slipping is not applied to said signals R and B giving rise to non-zero levels for the color difference signals (R-Y) and (B-Y), whereby the obtained image does not become white when CCD is saturated.
Furthermore, the combination of mosaic color filter with frame-transfer CCD, though allowing to avoid the drawback of color mixing, will lead to the formation of false color signals under certain conditions.